1. Field of the Invention
The present invention relates to an electrophotographic black toner for use in an image forming method applied to a copying machine and a printer which utilize or apply an electrophotographic process, as well as to an electrophotographic developer and an image forming method using said electrophotographic black toner. More particularly, the present invention relates to an electrophotographic black toner for use in a multicolor image forming method applied to a digital copier which forms a latent image with a laser beam, as well as to an electrophotographic developer and an image forming method using said electrophotographic black toner.
2. Description of the Related Art
In an electrophotographic developing process, a black toner comprising a dispersion of a non-magnetic black pigment such as carbon black in a binder resin is widely used as a developer. In conventional electrophotographic processes, the methods for developing and making visible a latent image formed on a photoconductive photosensitive material with a toner are generally classified into two-component developing methods and one-component developing methods. In the two-component developing methods, friction is caused between a black toner and a carrier to induce an opposite charge on the black toner, allowing adhesion of the black toner to a surface having a latent image by electrostatic attraction, whereby the latent image is developed. On the other hand, in the one-component developing methods, a thin toner layer is formed on a developing roll to cause a latent image to become visible. Since the one-component developing methods which require no carrier obviate the need for controlling the density of black toner in the developer, developing devices used in these methods are simple in structure and can be made compact. However, advanced techniques are required for the one-component developing methods to achieve performances equal to those attained by the two-component developing methods. As one of the one-component developing methods, there is known a so-called insulating non-magnetic toner development in which a magnetic particulate powder is not used but an insulating or highly resistant black toner, comprising a dispersion of a fine-particulate carbon black powder in a binder resin, is used.
In the currently most common PPC-type copier, a black toner for use in the two-component developing methods and the insulating non-magnetic toner development, both described above, is required to be insulating or highly resistant and have a volume-specific resistance value of 1012 xcexa9xc2x7cm or higher.
As stated above, the volume-specific resistance value of 1012 xcexa9xc2x7cm or higher is essential for the insulating or highly resistant black toner so as to retain the charge level high enough to develop the latent image. When the volume-specific resistance value is low, the toner may not retain an appropriate amount of charge because the charge is leaked away from the toner. Also the amount of charge may decrease because a charge of an opposite polarity may be induced. To suppress these phenomena, the insulating or highly resistant black toner is strongly required to achieve a volume-specific resistance value of 1012 xcexa9xc2x7cm or higher so that an appropriate amount of charge can be retained. If the amount of charge is small, attraction between a toner and a carrier is weak, and therefore in such cases as where a development area undergoes stirring or a mechanical impact is generated on a photosensitive material, the toner detaches from the carrier to thereby cause high background. In contrast, if the amount of charge is large, the toner tends to remain in the vicinity of the carrier whereby a decreased amount of toner migrates to the photosensitive material to lower the image density.
It is important for the carrier used in the two-component developing methods to be such that appropriate chargeability (in view of the amount and distribution of charge) is imparted to the toner, the toner retains suitable chargeability for long periods, and the toner is maintained such that the chargeability is not changed even when humidity and temperature change. To this end, various coated carriers which are surface coated with a resin has been proposed. Further in recent years, in order to achieve a higher quality image and improve reproducibility of a solid image, it has been proposed in Japanese Patent Laid-Open (JP-A) Nos. 1-101560 and 1-105264 to disperse a conductive material in a coating film to reduce the volume-specific resistance value of the carrier. However, if the volume-specific resistance value of the carrier is decreased, the resistance of a developer in which a mixture of a toner and the carrier exists is also decreased, whereby an opposite charge (a polarity opposite of the suitable polarity of the toner) is induced on the toner by the electrical field via the carrier during development. As a result, high background occurs since chargeability of the toner is lowered or polarity of the toner is opposite of the suitable polarity. To make matters worse, another problem arises that a copy produced by a copying machine first used after the machine was left unused overnight induces high background since charge leakage occurs and consequently the amount of charge is decreased.
As described above, in order to retain the charge level, the insulating or highly resistant black toner is required to achieve a sufficient insulating property, specifically, a volume-specific resistance value of 1012 xcexa9xc2x7cm or higher is required. In other words, even when a larger amount of black pigment is included in a black toner to enhance blackness, the black toner is required to suppress lowering of the charge level. That is, in order to maintain the volume-specific resistance value of the black toner as high as possible, the black pigment is also required to have a volume-specific resistance value as high as possible.
Currently, as the black pigment, a fine-particulate carbon black powder is mainly used in the black toner (see JP-A Nos. 4-142561 and 10-39546). However, when the fine-particulate carbon black powder was used to prepare a black toner having a volume-specific resistance value of 1012 xcexa9xc2x7cm or higher, there arose a problem that because the powder exhibited conductivity, the amount of it used was limited and a sufficient degree of blackness could not be obtained. Since the fine-particulate carbon black powder is conductive by itself and has a volume-specific resistance value of 1012 xcexa9xc2x7cm or smaller, when a large amount of the powder is used to enhance blackness, the volume-specific resistance of the black toner is decreased, making use as the insulating or highly resistive toner impossible. Further, although details are not yet elucidated, the toner containing a fine-particulate carbon black powder allows leakage of a relatively large amount of charge as described above, and is likely to cause high background even when the toner has the volume-specific resistance value of 1012 xcexa9xc2x7cm or higher. When the toner surface is viewed microscopically, it can be presumed that this is caused because the carbon black itself is conductive, and thus the charge of toner easily migrates.
Another example of black pigment used in a black toner is a hematite particulate powder containing Mn (see JP-A No. 10-279314). This particulate powder has a high volume-specific resistance value of 1xc3x97106 to 1xc3x97108 xcexa9xc2x7cm. However, its hue which ranges from reddish brown to dark brown does not achieve a sufficient degree of blackness. Even when formed into a toner, the hematite particulate powder exhibits a similar hue, and does not obtain a sufficient degree of blackness. If the toner contains a large amount of the hematite particulate powder, a certain degree of blackness can be obtained, but the volume-specific resistance value of the toner decreases.
Some proposals have been made to produce a toner in which carbon black and magnetite particles (having a hematite structure) are co-existent (see JP-A Nos. 3-056973, 6-067471, and 9-138527). JP-A Nos. 3-056973 and 9-138527 disclose a toner produced by using particles having a strong magnetic force, aiming at prevention of toner scattering from a developer holding member by increasing a constraining force between a carrier and the toner, and the force between the developer holding member and the toner. However, in such a toner, magnetic force is too strong and the amount of toner necessary to develop the image is decreased. JP-A No. 6-067471 discloses a toner whose chargeability has been improved. However, since the toner contains carbon black, the above-described charge leakage occurs when the toner is left unused, i.e., the charge level lowers even if the toner and carrier bear sufficient charge. Due to the above, for example, a first copy left overnight has a problem that high background occurs.
In the case of the two-component developer used in the two-component developing methods, since stirring is provided to a toner and the carrier to triboelectrically charge the toner, the amount of triboelectric charge of the toner can be controlled to a certain extent by selecting properties of the carrier and stirring conditions. Therefore, reliability in image quality is high and excellent. However, since the fine-particulate carbon black powder permits leakage of a relatively large amount of charge as described above, the toner produced using the fine-particulate carbon black powder tends to induce high background. This tendency is particularly notable when the toner is used in combination with a carrier having a relatively low resistance.
Therefore, there exists a great need for a black pigment which has a volume-specific resistance value high enough to be usable in a black toner and can suppress lowering of the charge level of black toner even when the toner contains a large amount of the black pigment. However, such a black pigment exhibiting such properties has not yet been obtained.
Therefore, an object of the present invention is to offer a solution to the above-described problems of the prior art and achieve the following goals. That is, the objects of the present invention are to provide an electrophotographic black toner which has a high volume-specific resistance value, exhibits a sufficient degree of blackness, is less likely to cause high background, and produces a high quality image, as well as to provide an electrophotographic developer and an image forming method using said electrophotographic black toner.
Through intensive research, the present inventors have solved the above-described problems. That is, the present invention provides the following  less than 1 greater than  to  less than 3 greater than .
 less than 1 greater than  An electrophotographic black toner comprising a colorant and a binder resin, wherein the toner has a metal oxide as the colorant of 20% by weight or less, said metal oxide having magnetization of 40 emu/g or smaller, and said toner has color coordinates such that L+has a value of 10 to 25, a* has a value of xe2x88x923.0 to 3.0, and b* has a value of xe2x88x923.0 to 3.0 as determined by a fixed image formed with the toner.
 less than 2 greater than  An electrophotographic developer comprising an electrophotographic black toner and a carrier, wherein the electrophotographic black toner described in  less than 1 greater than  above is used.
 less than 3 greater than  An image forming method comprising: a charging step for charging the surface of a latent image holding member uniformly; an exposing step to form an electrostatic latent image on the latent image holding member; a developing step for developing the electrostatic latent image with a developer on a developer holding member to form a toner image; a transferring step for transferring the toner image onto a transfer member; and a fixing step for fixing the toner image to the transfer member; wherein the electrophotographic black toner described in  less than 1 greater than  above is used.